Allow optimizing u32::from::<char>.

[reitermarkus]

Extracted from #124307.

This allows optimizing the panicking branch in the escape_unicode function, see https://rust.godbolt.org/z/61YhKrhvP.

[rustbot]

r? @workingjubilee

rustbot has assigned @workingjubilee.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[reitermarkus]

r? @joboet

[the8472]

You could use a transmute, it already emits the right assume in optimized builds

https://rust.godbolt.org/z/r7sqdcdzh

[joboet]

Arguably this should be done in the backend for char as u32 so that everyone can benefit from it.

[reitermarkus]

Arguably this should be done in the backend for char as u32 so that everyone can benefit from it.

Agreed, however I don't know where the code for that lives.

[clubby789]

That would be

rust/compiler/rustc_codegen_ssa/src/mir/rvalue.rs

Lines 529 to 531 in cb93c24

	(CastTy::Int(i), CastTy::Int(_)) => {
	bx.intcast(llval, ll_t_out, i.is_signed())
	}

[joboet]

I don't feel comfortable reviewing compiler changes, so
r? compiler

That said, I think this is a good idea!

[bors]

☔ The latest upstream changes (presumably #123886) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust.git master
$ git push --force-with-lease

The following commits are merge commits:

• 526aa8c

[clubby789]

Probably best to squash the first two commits, since they add then revert the library change

[scottmcm]

Thanks, this is looking good! One more request, then r=me.

[reitermarkus]

@scottmcm, I think changing the test to use ilog2 broke it. It still seems to work for each of the test functions individually, but if both are in the same file, it fails.